We’re trying to figure out why my brother’s boost keeps dropping after 6500rpms. He’s running an XS TO4S. Last week on the dyno it boost to 20 and then drops to 16 after 6K. He was running a 10psi spring which we think was the problem since boost controllers can only add about 70% of the spring (according to Apexi). So now he installed a 14psi spring. The drop in boost is less but still drops. It’ll boost up to 20 then drop to 18 after 6500. At first I though we were running out of turbo but then realize the wg is still opening all the way to redline. We’ve tried both the Greddy Profec B controller as well as a Blitz DSBC Spec R. Both the same.

 

Is the exhaust system restrictive ?.

 

would a restrictive exhaust cause boost drop? Even when I can hear the wastegate open?

I did make a resonated mp for his car a couple weeks ago